1. Field of the Invention
The present invention relates to a method for purifying a resin for photolithography, and a method for producing a resin for photolithography including the purification method. More specifically, the present invention relates to a method for purifying a resin for photolithography including an operation for purifying a polymerization solution after polymerization reaction and a method for producing a resin for photolithography including the purification method.
2. Description of Related Art
In photolithography, formation of a finer pattern is increasingly demanded due to increase in the integration density, and, at present, photolithography techniques with KrF excimer laser light (with a wave length of 248 nm) or ArF excimer laser light (with a wavelength of 193 nm) are used for mass production. A fine processing in an order of about not more than 50 nm is said to be possible by combining an excimer laser and an immersion lithography technique. Further, research and development are in progress also for photolithography techniques using F2 excimer laser light, which has a shorter wavelength (a wavelength of 157 nm), EUV (extreme ultraviolet) and X-ray, which have still shorter wavelengths than those excimer lasers, and the electron beam.
As miniaturization of the patterns proceeds, a higher quality has been demanded for a resin to be used for photolithography. For example, in cases where the resin contains low molecular weight impurities such as an unreacted monomer or a compound derived from polymerization initiator, the impurities cause a development defect of a resist pattern or deterioration of the storage stability of the resin, and are desired to be removed. As a method of removing low molecular weight impurities, a method (JP-A-2005-002236 (Patent Document 1)) is known wherein a reaction solution containing an acid decomposable resin having an alicyclic group obtained by radical polymerization reaction is brought into contact with a solvent which hardly dissolves or does not dissolve the resin in an amount less than five times the amount of the reaction solution by volume to precipitate a resin solid. Also known is a method (JP-A-2006-070167 (Patent Document 2)) wherein a polymer containing a monomer unit having a group exhibiting alkali-soluble function by partial detachment thereof by an acid is subjected to precipitation by an organic solvent, then further to a re-precipitation by aqueous solvent, a rinsing or repulping operation. Further, a method (JP-A-2008-127462 (Patent Document 3)) wherein, to a solution (a) containing a resin whose solution rate with respect to an alkaline developer is increased by the effect of an acid, a solvent (b) containing a poor solvent is added to prepare a solution (c), and then, the solution (c) is added to a solvent (d) containing a poor solvent to precipitate a resin whose solution rate with respect to an alkaline developer is increased by the effect of an acid (JP-A-2008-127462 (Patent Document 3)) or the like is known. In the methods of Patent Document 1 and Patent Document 2, however, the capability of removing low molecular weight impurities is not sufficient. In order to compensate for the insufficiency, since increase in frequency of purification or the like is needed, there are problems in that, for example, this method increases the manufacturing cost such as the number of manufacturing steps or the amount of solvent to be used. The method of Patent Document 3 include a step wherein, to a polymerization solution, a poor solvent is dropped. In such an operation wherein, to a large amount of resin solution, a poor solvent is added by a method such as dropping, a viscous resin which is hard to be confirmed by visual inspection may deposit and adhere to a wall of a container or the like.